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Date:   Mon, 21 Mar 2022 15:17:54 -0400
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Subject: Re: [PATCH v9 05/14] mm: multi-gen LRU: groundwork
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To:     Justin Forbes <jforbes@fedoraproject.org>,
        Yu Zhao <yuzhao@google.com>
Cc:     Andi Kleen <ak@linux.intel.com>, kernel-team@lists.ubuntu.com,
        Vaibhav Jain <vaibhav@linux.ibm.com>,
        Rik van Riel <riel@surriel.com>,
        Mel Gorman <mgorman@suse.de>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Aneesh Kumar <aneesh.kumar@linux.ibm.com>,
        Brian Geffon <bgeffon@google.com>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        Jesse Barnes <jsbarnes@google.com>,
        Sofia Trinh <sofia.trinh@edi.works>,
        "Huang, Ying" <ying.huang@intel.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Steven Barrett <steven@liquorix.net>,
        Shuang Zhai <szhai2@cs.rochester.edu>,
        Donald Carr <d@chaos-reins.com>,
        Oleksandr Natalenko <oleksandr@natalenko.name>,
        =?UTF-8?Q?Holger_Hoffst=c3=a4tte?= <holger@applied-asynchrony.com>,
        Will Deacon <will@kernel.org>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Mike Rapoport <rppt@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
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        Daniel Byrne <djbyrne@mtu.edu>,
        the arch/x86 maintainers <x86@kernel.org>,
        Konstantin Kharlamov <Hi-Angel@yandex.ru>,
        Matthew Wilcox <willy@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Michael Larabel <Michael@michaellarabel.com>,
        Linux-MM <linux-mm@kvack.org>,
        Kernel Page Reclaim v2 <page-reclaim@google.com>,
        Jan Alexander Steffens <heftig@archlinux.org>,
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References: <20220309021230.721028-1-yuzhao@google.com>
 <20220309021230.721028-6-yuzhao@google.com>
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From:   Prarit Bhargava <prarit@redhat.com>
In-Reply-To: <CAFxkdAouXr5Qn9asFge0P-HqA4aAk56yqP4xEiaWYRioJ1ssyA@mail.gmail.com>
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On 3/21/22 14:58, Justin Forbes wrote:
> On Mon, Mar 14, 2022 at 4:30 AM Yu Zhao <yuzhao@google.com> wrote:
>>
>> On Mon, Mar 14, 2022 at 2:09 AM Huang, Ying <ying.huang@intel.com> wrote:
>>>
>>> Hi, Yu,
>>>
>>> Yu Zhao <yuzhao@google.com> writes:
>>>> diff --git a/mm/Kconfig b/mm/Kconfig
>>>> index 3326ee3903f3..747ab1690bcf 100644
>>>> --- a/mm/Kconfig
>>>> +++ b/mm/Kconfig
>>>> @@ -892,6 +892,16 @@ config ANON_VMA_NAME
>>>>          area from being merged with adjacent virtual memory areas due to the
>>>>          difference in their name.
>>>>
>>>> +# the multi-gen LRU {
>>>> +config LRU_GEN
>>>> +     bool "Multi-Gen LRU"
>>>> +     depends on MMU
>>>> +     # the following options can use up the spare bits in page flags
>>>> +     depends on !MAXSMP && (64BIT || !SPARSEMEM || SPARSEMEM_VMEMMAP)
>>>
>>> LRU_GEN depends on !MAXSMP.  So, What is the maximum NR_CPUS supported
>>> by LRU_GEN?
>>
>> LRU_GEN doesn't really care about NR_CPUS. IOW, it doesn't impose a
>> max number. The dependency is with NODES_SHIFT selected by MAXSMP:
>>      default "10" if MAXSMP
>> This combined with LAST_CPUPID_SHIFT can exhaust the spare bits in page flags.
>>
>> MAXSMP is meant for kernel developers to test their code, and it
>> should not be used in production [1]. But some distros unfortunately
>> ship kernels built with this option, e.g., Fedora and Ubuntu. And
>> their users reported build errors to me after they applied MGLRU on
>> those kernels ("Not enough bits in page flags"). Let me add Fedora and
>> Ubuntu to this thread.
>>
>> Fedora and Ubuntu,
>>
>> Could you please clarify if there is a reason to ship kernels built
>> with MAXSMP? Otherwise, please consider disabling this option. Thanks.
>>
>> As per above, MAXSMP enables ridiculously large numbers of CPUs and
>> NUMA nodes for testing purposes. It is detrimental to performance,
>> e.g., CPUMASK_OFFSTACK.
> 
> It was enabled for Fedora, and RHEL because we did need more than 512
> CPUs, originally only in RHEL until SGI (years ago) complained that
> they were testing very large machines with Fedora.  The testing done
> on RHEL showed that the performance impact was minimal.   For a very
> long time we had MAXSMP off and carried a patch which allowed us to
> turn on CPUMASK_OFFSTACK without debugging because there was supposed
> to be "something else" coming.  In 2019 we gave up, dropped that patch
> and just turned on MAXSMP.
> 
> I do not have any metrics for how often someone runs Fedora on a
> ridiculously large machine these days, but I would guess that number
> is not 0.

It is not 0.  I've seen data from large systems (1000+ logical threads) 
that are running Fedora albeit with a modified Fedora kernel.

Additionally the max limit for CPUS in RHEL is 1792, however, we have 
recently had a request to *double* that to 3584.  You should just assume 
that number will continue to increase.

P.


> 
> Justin
> 
>> [1] https://lore.kernel.org/lkml/20131106055634.GA24044@gmail.com/
>>
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